Because of this double-function of the two-wire line and the associated cost saving, two-wire measurement apparatuses are being applied to an increasing extent in industrial process technology. An important industry standard in this connection is the ISA-A 50.1-Standard, in which direct current values between 4 mA and 20 mA characterize the particular measurement value and are transmitted over the two-wire line.
Not quite problem free in the case of two-wire measurement apparatuses is that even in the case of a very small current level of e.g. 4 mA there still must be enough power made available over the two-wire line to operate the measurement circuit, or its individual components. The power supply issue here is naturally even more critical, the higher the power requirement of the measurement circuit, or its components, becomes.
Basically, there are two possibilities for handling the problem. Either the measurement circuit is constructed of components with a correspondingly small energy consumption—a solution, which enables a continuous operation of the measurement circuit, or components with a relatively high energy consumption are operated intermittently. In the case of intermittent operation, energy is consumed only during the so-called active phase, while the recovery phase following thereafter is used for charging an energy storer, which then can supply the active components of the measurement circuit with the required power again in the next active phase. As an example of a two-wire measurement apparatus of the first named kind, U.S. Pat. No. 5,672,975 is noted. As an example for a device with intermittent operation, European Patent EP 0 687 375 B1 is noted named.
In the European patent, the measurement frequency of the measurement value transmitter is so designed, that the corresponding power demand is greater than the power available over the two-wire line in the case of minimum current and minimum voltage. Since the consumed power exceeds the available power during the operation of the measurement value transmitter, a deficit appears inevitably in the power balance. As soon as a sensing circuit recognizes a deficit, the measurement circuit stops operation of the measurement program, until the deficit no longer exists. In short, in this known solution, a deficit is diagnosed in an energy storer. On the basis of this deficit, a longer cycle time is predicted to be necessary—the measurement frequency is then correspondingly changed. In the end, this means that it is always estimated, when the energy storer will be completely charged, or charged to a certain level. Following expiration of this estimated time, a measurement signal is then issued. This known solution has the disadvantage, that it accepts excessively long inactive phases. Consequently, the measurement rate of the measurement system—and thus the measurement accuracy of the fill level measurement apparatus—is lowered.